1. Field of the Invention
The present invention relates to high-frequency semiconductor devices. The present invention more particularly relates to a high-frequency field effect transistor (FET) for use in a millimeter-wave or quasi-millimeter-wave circuit module for amplification, oscillation, or modulation.
2. Description of the Related Art
FIG. 1 is a plan view showing a structure of an electrode formed on a semiconductor surface in a currently used conventional high-frequency FET. In this high-frequency FET, three source electrodes 2 extend from a source pad section 1 and two drain electrodes 4 extending from a drain pad section 3 are disposed between the source electrodes 2. Four very narrow gate electrodes 6 extending from two gate pad sections 5 disposed between the tips of the drain electrodes 4 and the source pad section 1 extend the length of areas sandwiched by the source electrodes 2 and the drain electrodes 4. In other words, this high-frequency FET is a horizontal-type (plane-type) FET in which the source electrodes 2, the gate electrodes 6, and the drain electrodes 4 are formed on the same plane.
To make such an FET usable at higher frequencies, it is necessary to reduce the distance between the source electrode and the drain electrode and to narrow the gate electrode (reduce the gate length). A narrow and long gate electrode has large parasitic resistance and parasitic capacitance, however, and thereby the characteristics of the FET deteriorate, including for example a noise increase, an operating-frequency decrease, a gain reduction, and an increased input/output reflection loss.
When the FET is viewed as a waveguide which transmits a microwave, it has a very unusual structure and its operating range is limited to a low frequency zone in which the FET can be approximated to a lumped-constant circuit device.
FIG. 2 is a plan view illustrating an electrode structure of an air-bridge-gate-structure FET which improves upon the above FET of FIG. 1 and has less characteristics deterioration. In this structure, a source electrode 8 disposed between source pad sections 7 on a semiconductor surface faces a drain electrode 9 along its full length. A wide gate electrode 11 extends over the source electrode 8, a gate pad section 10 is disposed at the side of the gate electrode opposite the drain electrode 9 against the source electrode 8, and the distal edge of the gate electrode 11 is Schottky-connected to the semiconductor surface between the source electrode 8 and the drain electrode 9.
Since the gate electrode 11 can be made wide in such an air-bridge-gate-structure FET, the parasitic resistance and the parasitic inductance of the gate electrode 11 are reduced and the RF characteristics (especially noise characteristics) are improved.
At a portion where the gate electrode 11 passes over the source electrode 8, a parasitic capacitor is generated between the source electrode 8 and the gate electrode 11, which decreases the operating frequency. To reduce this parasitic capacitance, the source electrode 8 needs to be narrowed. If the source electrode 8 is narrowed, the source electrode 8 gains additional parasitic resistance and additional parasitic inductance, so only a limited amount of characteristics improvement is available in such a method.
When this air-bridge-gate-structure FET is viewed as a waveguide, it has a very unusual structure, like a horizontal-type FET, and its operating range is limited to a low frequency zone in which the FET can be approximated to a lumped-constant circuit device.
To solve the problem of characteristics deterioration in a high-frequency FET for use in millimeter-wave and quasi-millimeter-wave ranges, it is an important issue to eliminate wiring resistors and parasitic components such as parasitic capacitors and parasitic inductors in the gate electrode and the drain electrode, as described above. It is very difficult, however, to suppress characteristics deterioration in a high-frequency range in a conventional FET structure, and a semiconductor device suited to a millimeter wave and a quasi-millimeter wave has not yet been manufactured.